Recent advances have been made in the field of producing prototype parts, or finished parts in small quantities, directly from computer-aided-design (CAD) data bases. An important one of these advances produces parts from a powder in layerwise fashion by scanning selected portions of a layer of powder with an energy beam, such as a laser. The energy from the beam fuses the powder at the scanned locations into a mass within the layer which adheres to portions of previously fused layers. The fused portions of each powder layer are defined according to a cross-section of the part, so that a series of layers processed in this manner results in a finished part. As a computer can control the scanning of the energy beam, this method can directly transfer a design from a CAD data base into an actual part.
This method, and apparatus for performing the same, are described in further detail in U.S. Pat. No. 4,863,538, issued Sep. 9, 1989, U.S. Pat. No. 5,017,753 issued May 21, 1991, U.S. Pat. No. 4,938,816 issued Jul. 3, 1990, and U.S. Pat. No. 4,944,817 issued Jul. 31, 1990, all assigned to Board of Regents, The University of Texas System and incorporated herein by this reference. Parts are now being commercially produced according to this method, namely the "SELECTIVE LASER SINTERING" method, by the SLS Model 125 DeskTop Manufacturing system manufactured by DTM Corporation of Austin, Tex.. As described in the above-noted patents, and also in U.S. Pat. No. 5,156,697 issued Oct. 20, 1992, U.S. Pat. No. 5,147,587 issued Sep. 15, 1992, and U.S. Pat. No. 5,182,170 issued Jan. 26, 1993, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, various materials and combinations of materials can be processed according to this method, such materials including plastics, waxes, metals, ceramics, and the like. In addition, as described in these patents and applications, the parts produced according to the "SELECTIVE LASER SINTERING" method can have shapes and features which are sufficiently complex as to not be capable of fabrication by conventional subtractive processes such as machining.
Other additive methods also fuse selected portions of a layer of powder in the layerwise formation of a three-dimensional part. An example of such a method is described in Sachs, et al. "Three Dimensional Printing of Ceramic Shells and Cores for Metal Casting", Proc. of the 39th Annual Technical Meeting: Investment Casting Institute (1991), pp. 12:1-12:14. In this method, a layer of powder, such as a ceramic powder, is dispensed and a binder material is applied to selected portions of the powder, for example by way of an ink-jet printhead. In this method, the locations of the powder layer that receive the binder are defined according to a cross-section of the part to be produced, communicated to the apparatus by a CAD data base.
In these methods, as well as in any additive manufacturing method using layers of powders, the proper delivery of powder is a critical factor. The proper volume of powder must be dispensed in each layer so that the fused portion both adheres to previously fused layer portions, and also provides a fused mass of the proper dimensions to which succeeding fused layers can adhere. In addition, particularly where mixtures of multiple materials are used, good homogeneity and physical uniformity of the powder (i.e., powder particles of uniform size, with no caking or clumping) is necessary for proper part fabrication. Where the part to be produced is a finished part, rather than a form factor model of the part, the powder must also have sufficient density to provide the necessary structural strength.
A roller for properly delivering a layer of powder over a laser target area in the "SELECTIVE LASER SINTERING" process is described in the above-referenced U.S. Pat. No. 5,017,753 issued May 21, 1991, and incorporated herein by reference. As described therein, a counter-rotating roller is used to smoothly spread a volume of powder of the proper thickness over the target area.
The SLS Model 125 DeskTop Manufacturing system manufactured by DTM Corporation incorporates the counter-rotating roller in combination with a dual piston system. In the SLS Model 125 system, one piston is used for lowering the fused and unfused powder in the target area during the fabrication process, so that the top surface of the target area remains at the same distance from the laser. A second piston is located alongside the part piston for providing unfused powder to the process. In operation, the powder piston is first raised a specified amount (e.g., on the order of 0.010 inches) and the part piston is lowered a specified amount (e.g., on the order of 0.005 inches). The counter-rotating roller is moved across the raised surface of the powder piston, pushing the powder to leave a layer over the lowered surface of the part piston. In this way, the SLS Model 125 system provides a measured volume of powder for each layer of the part to be produced.
Referring to FIG. 1, an example of such a prior apparatus for producing parts in layerwise fashion will now be described. The apparatus shown in FIG. 1 is a schematic representation of the SLS Model 125 DeskTop Manufacturing system. The apparatus of FIG. 1 includes a chamber 2 (front doors and the top of chamber 2 are not shown in FIG. 1, for purposes of clarity), within which the selective sintering process takes place. Target surface 4, for purposes of the description herein, refers to the top surface of heat-fusible powder (including portions previously fused, if present) disposed on part piston 6. The vertical motion of part piston 6 is controlled by motor 8. Laser 10 provides a beam which is reflected by galvanometer-controlled mirrors 12 (only one of which is shown for clarity), in the manner described in the above-incorporated U.S. Patents. Powder piston 14 is also provided in this apparatus, controlled by motor 16. As described in the above-referenced U.S. Pat. No. 5,017,753, counter-rotating roller 18 is provided to transfer the powder to the target surface 4 in a uniform and level fashion. The surface of roller 18 is preferably knurled, or otherwise textured, as described therein.
In operation, the apparatus of FIG. 1 supplies powder to chamber 2 via powder cylinder 14; powder is placed into chamber 2 by the upward partial motion of powder cylinder 14 provided by motor 16. Roller 18 (preferably provided with a scraper to prevent buildup, said scraper not shown in FIG. 1 for clarity) spreads the powder within the chamber by its translation from the location of powder cylinder 14 toward and across target surface 4 at the surface of the powder on top of part piston 6, in the manner described in said U.S. Pat. No. 5,017,753. At the time that roller 18 is providing powder from powder piston 14, target surface 4 (whether a prior layer is disposed thereat or not) is preferably below the floor of chamber 2 by a small amount, for example 5 mils, to define the thickness of the powder layer to be processed. It is preferable, for smooth and thorough distribution of the powder, that the amount of powder provided by powder cylinder 14 be greater than that which can be accepted by part cylinder 6, so that some excess powder will result from the motion of roller 18 across target surface 4; this may be accomplished by the upward motion of powder piston 14 by a greater amount than the distance below the floor of chamber 2 to which target surface 4 is lowered (e.g., 10 mils versus 5 mils). It is also preferable to slave the counter-rotation of roller 18 to the translation of roller 18 within chamber 2, so that the ratio of rotational speed to translation speed is constant.
Further in operation, after the transfer of powder to target surface 4 and the return of roller 18 to its original position near powder piston 14, laser 10 selectively sinters portions of the powder at target surface 4 corresponding to the cross-section of the layer of the part to be produced, in the manner described in the above-referenced U.S. Patents. The sintering is preferably performed in alternating directions, as described in U.S. Pat. No. 5,155,324 issued Oct. 13, 1992, and incorporated herein by this reference. After completion of the selective sintering for the particular layer of powder, part piston 6 moves downward by an amount corresponding to the thickness of the next layer, awaiting the deposition of the next layer of powder thereupon from roller 18.
One or more radiant heaters (not shown) are suspended from the roof of chamber 2 (in a manner not shown); the preferred shapes of such radiant heater include a ring, a partial cone, or flat panels. U.S. Pat. No. 5,155,321 issued Oct. 13, 1992 assigned to DTM Corporation and incorporated herein by this reference, describes the preferred implementation of such radiant heaters. As disclosed therein, radiant heat applied to the target surface assists in maintaining temperature uniformity so that a high precision part may be formed. Furthermore, gas flow distribution may also be provided within chamber 2, for example by way of a baffle and vents (not shown), also for maintaining thermal uniformity in the part being formed. Such gas flow distribution is described in U.S. Pat. No. 5,155,321 assigned to DTM Corporation and incorporated herein by this reference.
In the prior arrangement of FIG. 1, a single powder piston 14 is provided, and located along one side of target surface 4 at the top surface of part piston 6. As such, the powder is delivered in a single direction, with any excess powder remaining in chamber 2 on the side of part piston 6 opposite powder piston 14. Periodic cleaning and recovery of such excess powder is therefore required. Furthermore, the process of producing parts is slowed by the necessity of roller 18 to return to its prior position after each layer of powder is applied over target surface 4. It is contemplated that the provision of powder over target surface 4 will become an even greater portion of the overall fabrication time for a part as the laser power and optics, and also thermal control, continues to improve.
It is therefore an object of this invention to provide a powder delivery system which provides improved powder utilization and reduced powder loss.
It is a further object of this invention to provide such a system which facilitates the installation of unfused powder prior to production of the part, and also the removal of powder which is not fused during such production.
It is a further object of this invention to provide such a system in which multiple types of powders may be delivered in alternating layers.
It is a further object of this invention to provide such a system having improved efficiency of production due to improved efficiency powder delivery.
Other objects and advantages will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.